DESCRIPTION (taken from the application) In 1997, there were an estimated 16,100 new cases of thyroid cancer in the United States. Cancer is not simply a proliferation process, but the manifestation of an imbalance between cell growth and cell death. It is likely that for a tumor clone to progress the apoptotic program must be successfully disabled. In support of this paradigm, we have isolated a chimeric and truncated mutant of PKC-epsilon (Tr-PKC-epsilon), the gene for which was amplified and rearranged in a thyroid cancer cell line. When transfected into PCCL3 cells (a well-differentiated rat thyroid cell line) Tr-PKC-epsilon inhibits activation-induced translocation of the wild-type isozyme, resulting in protection of cells from apoptosis. This is accompanied by a marked impairment in p53 stabilization, which may be in part due to elevated levels of MDM2. These findings point to a role for PKC-epsilon in apoptosis signaling pathways in thyroid cells, and suggest that disruptions in PKC-epsilon function may be involved in thyroid tumorigenesis, possibly by altering the cellular response to DNA damage. In support of this we have found that in 75-85% of thyroid carcinomas there were dramatic changes in the level and/or subcellular distribution of PKC-epsilon compared to corresponding normal thyroid tissue. The following Specific Aims are proposed: (1) We will use an inducible expression system to achieve selective activation of either PKC-epsilon or the constitutively activated mutant PKC-epsilon-A159F and to determine whether this alone can initiate an apoptotic program, that can be blocked by Tr-PKC-epsilon. (2) We will explore whether PKC-epsilon activation interferes with phosphorylation, stabilization, and other post-translational modifications of p53 and MDM-2. (3) We will manipulate the function of the isozyme in thyroid follicular cells of transgenic mice, by targeting expression of either PKC-epsilon, PDK-epsilon-A159F, or the dominant negative inhibitor Tr-PKC-epsilon. Effects on thyroid cell apoptosis in vivo will then be studied in mice exposed to external radiation to the thyroid bed. (4) We will determine if the observed changes in expression and distribution of PKC-epsilon in thyroid cancers are due to somatically-acquired structural defects in the PKC-epsilon gene, or to epigenetic events. For tumor clones to expand, they must not only exhibit unrestrained stimulation to proliferate, but must also disable essential protection circuits that trigger apoptosis. We propose that PKC-epsilon is part of this defensive strategy, and that this can be subverted during tumorigenesis, or perhaps modulated during adaptive responses such as goiter involution, or thyroid remodeling.